A system for sensing perturbations in a distributed manner using an array of multimode fibres for localization (zone identification) of the perturbation. Particularly, the present invention relates to a two dimensional array based speckle pattern sensing system having 4xc3x973 (12) multimode fibres for sensing perturbation in multiple zones, said multimode fibres are imaged in parallel by a charge coupled device (CCD) camera and their speckle pattern analyzed by a conventional image processing hardware to determine the perturbations.
Various types of environment perturbation measurement have been attempted by researchers and that includes, but not limited to, displacement, vibration, pressure, stress, strain, temperature, intrusion, acoustic wave etc. The principle of speckle pattern and its use in sensing environment perturbation is well known. An analysis of the changes in the speckle pattern output from a multimode optical fibre could be used to obtain information about the perturbation of the fibre. In all the cases reported till now a single multimode fibre is used and a single speckle pattern analyzed for perturbation sensing. Although, the distribution sensing capability of the fibre can cover a large area of sensing, but the sensing zone remains only one. Therefore, the localization of the perturbation always remains a problem.
It is well known that the output speckle-intensity distribution from a multi-mode fibre is affected by the perturbation of the fibre, and it can be used for sensing. In other words, if the fibre status changes because of an external perturbation, the output speckle pattern also changes. Even though it is difficult to predict the speckle distribution, an approximate relationship between the perturbing factor and the speckle intensity distribution may be developed for which the perturbation of the fibre may be determined.
When the output from a multimode fibre is projected on a screen (e.g., a ground glass plate), a uniform circular pattern is observed. When the light is coherent, the pattern becomes very granular, consisting of a very large number of speckles of varying intensities with smooth distribution of intensity. The distribution of these speckle changes slowly over time, but the intensity of the total circular pattern remains basically constant. This pattern is very sensitive to perturbations (that include, but not limited to, displacement, vibration, pressure, stress, strain, temperature, intrusion, acoustic wave etc) on the fibre which generate the change in path of light in various modes. When the optical fibre carrying the coherent light is perturbed, the distribution of the speckle intensities is seen to change with the perturbation, with some speckles becoming brighter, some dimmer, and some not changing at all. The total intensity of the pattern remains unchanged, however. Hence it is not an intensity-modulated sensor as the total intensity is always a constant.
The system in its prior form is used as follows. The multimode fibre is kept in the environment to be sensed in a distributed manner, i.e. the changes in the perturbation any where on the length of the fibre can be sensed equally without discrimination. As mentioned earlier, the fibre may sense various types of environment perturbation. A CCD camera senses the speckle pattern generated at the end of the fibre core falling on a ground glass plate. The CCD camera has an array of photosensors (charge coupled) arranged in a two dimensional array form. Each photosensing element contributes one picture element, or pixel, to the image frame. The image falling on the CCD can be captured by image acquisition hardware residing in a computer. The amplitude of the perturbation is sensed by the image processing of the speckle pattern. The typical processing requirement of an image here are differentiating the current speckle image with respect to the previous or the reference speckle image stored earlier. In this manner the amplitude of the perturbation can be measured.
The above solution to perturbation sensing appears fine unless one realises that the present day standard cameras are capable of acquiring images of the order of 768xc3x97574 or more in the two dimensions. At the time when the experimental studies for such speckle image processing were being developed, it was found that the information describing an image can be unacceptably large for processing of speckle patterns in real time because of non availability of such real-time processing hardware. It was then shown by (Kulchin et. al., Optical Engg, V. 36, No. 5, 1997) that television standard images contain unnecessary information about the speckle pattern formed by ordinary multimode optical fibres. The average size of the speckles can be calculated as (Svelto, 1982):
Average size of the speckles=2R(xcex/D),
where D is the diameter of the source of light (optical fibre core), R is the distance between the source and plane of registration, and xcex is the wavelength of the laser diode.
The diameter of a light field formed by an optical fibre in the plane, placed at a distance R from its output, can be calculated as (1983, Synder and Love):
The diameter of light field=2.NA.R,
where NA is the numerical aperture of optical fibre. Thus, the entire number of light and dark speckles in the speckle pattern can be calculated by:
Total number of light and dark patterns=(NA2.D2)/xcex2
If formed by a standard multimode optical fibre with parameters NA=0.2 and D=50 xcexcm, and using a Laser Diode(xcex=0.6328 xcexcm), the number of such speckles in a circular pattern is ≈252. The standard CCD cameras with 756xc3x97582 pixels can easily sense the perturbation affecting the speckle distribution. The latest pipelined or parallel image processing hardware allows image frame operations in real-time. This forms the basis of the development of the device, which has even wider sensing ability by way of an array of multimode fibres.
Spillman et. al. (Applied Optics, V. 28, No. 15, 1989) claim the use of Statistical Mode Sensor (SMS) include, but are not limited to, intrusion detection, structural vibration sensing, and acoustic sensing. In their sensor, all the image (using 128xc3x97128 array of photodiodes) processing occurs in electronics. Just before a new pixel is stored in the frame buffer, the old pixel data is removed, and both old and new pixel data are passed to an arithmetic circuit. The arithmetic circuit finds the absolute value of the differences between the old and new pixels. All the absolute values of the differences for the entire frame are then accumulated and normalised to give a single value. The single value represents the amount of change in the speckle pattern that occurs over a period of time between the captured frames.
The main object of the present invention is to provide a system capable of sensing perturbation in multiple zones at the same time.
Another object of the present invention is to provide a multi-fiber optic 2D-array system for sensing and localizing environmental perturbations.
Still another object of the present invention is to provide a multi-fiber optic 2D-array system for sensing perturbations using speckle image processing.
Yet another object of the present invention is to provide a system wherein the sensing module comprises of a Multi Fiber Adapter (MFA), a diffusing glass plate and a lens assembly which can stand alone as a unit and be used as a sensing system for sensing as well as localization of perturbation in a distributed manner.